I'd like to make it so that the townies (or any Sim from a non-playable family) progress alongside my active family, but not my non-active households. Is such a thing possible and if so, how do you suggest I set it up?

We might be tripping over the concepts of "townie" vs. "non-playable" there. Sims that are not playable are not part of resident households, they are homeless. But is this the kind of thing you had in mind?
Story-Progression-Rotational-Caste-Settings

Ah.
Yes it is. Makes my head spin a bit, but that's what I'm looking for. Thanks.
Will other playable households that you haven't started yet be affected by Story Progression?

I think we might be approaching the game from different angles or something. All resident sims who are not and while they are not controlled by the player are affected by story progression, this is how TS3 is designed. There isn't any concept of started vs. non-started households, if they are living in town then something is controlling them. The only way a household can be not started is if they haven't been placed on the map yet.

The point of the Rotational Play strategy of StoryProgression is to restrict some (or all) resident sims from various specific aspects of progression while they are not being played and to keep their households together as we left them.

Under EA standard, TS3 is designed so that we play one household. The other residents around that one exist for the convenience and amusement of our one active family, we aren't supposed to care about what they do, what happens to them, or if they even get culled (removed from the game entirely) at all. EA provides us with a way to switch active households whenever we want, but then the ones we have left behind will do all kinds of unwanted things on us. Of course we are here using mods like SP because we want better than all that but sims have to live their lives while they are not being played even if they are restricted from progressing or reaching milestones in the players' absence. An example of a game where all sims in town are played entirely and really nothing happens at all while the player is hopping around to other households would be TS2 (and maybe TS4, wouldn't know from experience), but TS3 just doesn't work like that.

Ah. Okay. I will admit, I'm very new to TS3 (or playing The Sims in general).